slag/refractory and metal/refractory interactions during ... ?· slag/refractory and...

Download Slag/refractory and metal/refractory interactions during ... ?· SLAG/REFRACTORY AND METAL/REFRACTORY…

Post on 01-Apr-2019

222 views

Category:

Documents

2 download

Embed Size (px)

TRANSCRIPT

SLAG/REFRACTORY AND METAL/REFRACTORY INTERACTIONS 423

IntroductionColumbus Stainless is situated in Middelburg, South Africaand is a subsidiary of Acerinox SA. The steel-makingfacilities currently consist of a 100-t EAF (with sidetapping) two stationary 120-t bottom and top blownconverters (one in use while the other is re-lined), ladlerinsing station and a straight mould, curved bow,continuous casting machine. The product range coversstandard 300 and 400 grades as well as titanium-stabilizedgrades. The current throughput is 55 to 60 000-t per month,with 5% austenitic Ti-stabilized and 10% ferritic Ti-stabilized grades. More detail of the plant is givenelsewhere13.

The continual drive for increased throughput, improvedquality and lower costs of production, necessitates theunderstanding of numerous variables. Two of thesevariables, namely refractory/slag and refractory/steelinteractions, are the focus of this paper. Increased sequencelength results in longer exposure of refractory to steel orslag, the effect being more erosion and chemical attack,which may influence quality. Higher quality refractorymaterials result in increased costs, thus a balance needs tobe struck between costs, quality of the steel, andthroughput.

Refractory properties play a vital role in their behaviourwith slag and steel. These properties are both physical andchemical in nature, and extensive research has beenconducted to understand the various mechanisms at play inthe systems (refractory/steel and refractory/slag)410. Thepurpose of this work was to understand which mechanismsare at play within the system. The work also aimed todetermine criteria for selection of new refractory products.

The main area of study in this work was the tundish, interms of tundish working linings, tundish furniture andblack refractories. The steel from the ladle enters the 25 Ttundish via a refractory shroud with argon shieldingbetween the collector nozzle and shroud. There is an impactpad and baffle in the tundish itself, and steel flow out of thetundish is controlled via a stopper rod system. Figure 1shows the basic layout of the tundish and the position of the

furniture. A detailed description of the tundish design andfurniture set-up is given elsewhere3.

Materials and experimental techniquesThe first part of the work examined plant refractorysamples from the tundish (used baffles, impact pads, blackrefractories and tundish linings). Samples were collectedand studied under the SEM and the slag/refractory andsteel/refractory interface reactions described. Subsurfacetundish lining samples were studied to assess the level ofslag coating that occurs during tundish draining45. Thiscoating may then interact with the steel on the next heat andmay be a source of inclusions in the steel.

The technique utilized for the laboratory work, was asimple dip test whereby samples of refractory were held inmolten steel for pre-determined periods of time in aninduction furnace. The samples were then prepared andanalysed under the SEM (with EDS facility) to study theinterface between the steel and refractory. This techniqueallowed a detailed description of the interactions to be madeand also to determine the best refractory for the particularpurpose to be selected.

SUTCLIFFE, N. Slag/refractory and metal/refractory interactions during the production of stainless steels. VII International Conference on Molten SlagsFluxes and Salts, The South African Institute of Mining and Metallurgy, 2004.

Slag/refractory and metal/refractory interactions during the production of stainless steels

N. SUTCLIFFEColumbus Stainless, South Africa

Refractories and slags play a vital role in the production of stainless steel. They are in intimatecontact with the steel at temperature, and chemical reactions between the three can and do occur.These interactions can be beneficial, in the case of active tundish and ladle slags, and negative inthe case of refractory erosion and slag entrapment. This paper presents the results of both planttrials and laboratory investigations of steel/refractory and slag/refractory interactions. Theinteraction mechanisms observed are described leading to selection criteria for new refractoryproducts.

Keywords: slags, steels, refractories, reaction mechanisms.

Figure 1. Basic tundish design and positions of furniture

ShroudStopper

Impact Pad

Baffle

Grid Mar 20 2002FLUENT 5.5 (3d, segregated, ke)

Z

Y X

MOLTEN SLAGS FLUXES AND SALTS424

Refractory bar samples, approximately 20 x 50 mm(cross-section) and 120 mm in length were suspended in a20 kg induction furnace. The induction furnace was open tothe atmosphere, and the crucible was MgO based. Thesamples were pre-heated for one hour at 1280C and thentransferred to the induction furnace. The 409 alloy,chemical analysis in Table I, was held at 1600C. The highoperating temperature was utilized to ensure a severe attackwithin the ten-minute period of submergence. The short testperiod was utilized to limit the decarburization of therefractories and thus maintain a stable C level in the steel.The chemistry of the melt was analysed after each test andcorrected to ensure relatively stable experimentalconditions. The effects of chemical changes, such asdecarburization of the refractory, were not assessed in thiswork. After submergence, the samples were allowed to coolin air. The steel samples were ground on P80 grit Al2O3paper and polished on P180 grit Al2O3 paper and analysedby means of the ARL9800 XRF technique. The carbon wasanalysed by means of a Leco CS400 optical emissionspectrometer with a relative accuracy of 0.003 wt%. Theoxygen and nitrogen were analysed by means of a LecoT316 OES, with a standard deviation of 2 ppm.

Samples for SEM analysis were prepared to a 3 mmfinish using standard metallographic techniques and coatedwith a thin layer of gold for conductivity

Slag samples taken from the plant (Table I) were milledto a 150 mm particle size, then briquetted, and thechemistry was analysed with an XRF fluorescence method(ARL9800 wavelength dispersive instrument) to determinethe oxide compositions. The standard deviation for oxidesin the slag ranges between 0.1 and 1 wt% dependent uponthe particular oxide (CaO = 0.9%, Al2O3 = 0.3%, MgO =0.1%, SiO2 = 1%).

Wet chemical analysis of the un-reacted materials wasperformed via lithium tetraborate fusionICP/OEStechnique. Table II gives the chemical analysis of allrefractories utilized in the current study. Table III shows themechanical/physical properties of the refractories.

SEM/EDS analysis was conducted on all raw materialsand samples prior to the plant and laboratory tests. Themain phases present in the various products are shown inTable IV with Tables V and VI showing the post reactionphases observed.

Black refractoriesThree distinct types of black refractory were tested in thelaboratory:

Al2O3-SiO2-C refractoriesD99, D90, D63 and V101 MgO-C refractoryDM4 ZrO2-CZ6 and Z62

CastablesThe dip tests were utilized to study the MgO based castableproducts, Table II samples M1 and M2. The plant samplewas an Al2O3-SiO2 based product (L17), and was comparedin the un-reacted and reacted states with M1 and M2. L17 isutilized in the manufacture of both the impact pad andbaffle, a plant sample was taken and analysed after a 4-ladlesequence of 409 material.

LiningsThree types of tundish lining material were studied in thecurrent work. The first, T1, was a standard spray material;the second an active spray material applied on to thestandard spray material (T2); and thirdly, a dry-vibematerial (T3). Samples were taken from the tundish slagline area and from the steel/refractory contact area.

Sample C S Si Mn Cr Ni Co Ti N O Al Mo P

Lab 0.023 0.005 0.5 0.2 11.5 0.1 0.1 0.24 0.020 0.005 0.003 0.1 0.025Plant 0.011 0.002 0.55 0.3 11.6 0.15 0.1 0.18 0.010 0.003 0.005 0.1 0.019

Sample CaO MgO SiO2 TiO2 MnO Cr2O3 Al2O3Start 34.4 4.36 35.9 4.18 1.35 1.43 16.3End 34.5 3.98 27.0 17.2 1.33 1.35 15.8

Table I409 alloy chemistries (wt. %) and plant slag analyses

Sample Al2O3 TiO2 SiO2 CaO MgO ZrO2 Total C Other

D90 51.00 0.9 14.00 0.30 0.60 1.8 32.00D99 58.70 0.8 10.20 0.20 0.30 0.6 31.50DM4 2.20 - 6.30 1.60 69.0 0.3 16.00D63 61.50 0.8 10.00 0.20 0.3 4.5 24.50Z6 0.30 - 8.00 2.40 0.2 74.5 16.50Z62 0.30 - 7.00 2.00 0.2 75.0 14.50T2 0.82 0.01 4.35 21.60 65.3 0.935 Fe2O3 : 2, + BinderT3 1.70 92.0 3.2 Fe2O3 : 2, + BinderM1 0.22 10.50 2.08 88.4 Fe2O3 : 1L17 64.00 33.00 0.60 0.6 Fe2O3 : 0.5M2 0.60 0.2 5.90 2.60 90.7V101 52.00 15.70 31.20 Fe2O3 : 0.5, B2O5 : 1.6T1 1.50 0.5 5.00 2.00 87.2 Bal: Metal Oxides

Table IIChemical composition of refractory samples

SLAG/REFRACTORY AND METAL/REFRACTORY INTERACTIONS 425

Results and discussion

Black refractoriesThe Al2O3-SiO2-C refractories have a range of 51 to 61%Al2O3, 10 to 16% SiO2, and 24 to 3% C. The depth ofpenetration of the refractory by the steel is shown in Table VII. The direct affect of C content is notdeterminable from these tests, but can be inferred from thereaction products seen on the samples. Cirilli et al10 andKwon et al.11 discussed the reduction of refractory oxidesby carbon in the refractory, leading to changes in the rate ofC dissolution when a refractory is in contact with a steel attemperature. The final effect of the C dissolution is toproduce oxides in the steel, which are then deposited on therefractory. The proposed reactions are shown in Equations[1] to [5]. (The phase conventions used in the followingreactions are: refractory/slag component x; [i]component i in the steel; {y} gaseous component y.)

[1]

[2]

[3]

[4]

[5]

[6]

The products of reactions [1] and [2] were seen on

Recommended

View more >